This invention concerns a method and a device for effecting, in deviated well portions highly inclined to a vertical line or horizontal, such operations as measurements by means of specialized tools.
According to the prior art, specialized tools for effecting such operations as, for example, measurements, are secured to the end of a carrying wireline and moved into the well by the action of gravity without substantial difficulty as long as the inclination of the bore-hole with respect to a vertical line does not exceed about 45.degree.. Beyond this limit, the displacement of the tools is only possible when the profile of the bore-hole and the diameter variations thereof are known, and by making use of tools of a small size.
For highly inclined wells, it has been proposed in U.S. Pat. No. 4,168,747 to place in the well a flexible line provided at one end thereof with a head producing fluid jets which aid the progress of the flexible line through the well. The tool is introduced into the flexible line and its displacement is obtained by pumping the fluid filling the well and the flexible line. The tool which is maintained permanently inside the flexible line is, accordingly, necessarily of a type whose operation is not disturbed by the presence of the flexible line, for example, a neutronic or .gamma. rays sonde for measuring the characteristics of the formations.
Such a solution suffers from a number of drawbacks. As a matter of fact, not only it is not usable with all the tools which may have to be introduced into the well, such as an electric or electromagnetic sonde, but its operation is time consuming. Moreover, the unavoidable friction of the flexible line against the wall of the well, particularly in the highly inclined portions, requires, for its progression, very powerful jets which locally damage the borehole wall. Such a device cannot therefore be used for introducing tools in highly inclined portions of great length, and in substantially horizontal well portions.
It is also known to adapt a tool such as a measuring sonde at the end of a substantially rigid hollow string so as to move it by the action of a thrust exerted on the string.
The disadvantages of this solution result from the fact that the tools placed at the end of the string rub against the wall of the well and may be damaged. On the other hand, these tools are connected to the surface through a control and measurement signals transmission cable which is housed in the bore of the hollow string, thereby making substantially more complex the assembling of the end-to-end screwed elements forming the string.
In order to reduce the latter disadvantage, there can be used a special sub generally called in the art "Side Entry Sub" and described, for example, in U.S. Pat. No. 4,062,551.
The screwing or unscrewing of the string sections above this special sub is simplified in view of the fact that above this sub, the cable is external to the hollow string. Nevertheless, the above mentioned disadvantage continues to exist for the string section between this special sub and the tool. Moreover, care must be taken to avoid blocking of the cable which, above the special sub, is located in the annular space defined between the hollow string and the wall of the well.
The location of this special sub on the hollow string is preferably selected so that, during the displacement of the string, this sub remains in the vertical section of the borehole wall.
U.S. Pat. No. 4,039,237 describes a drilling apparatus whereby a cable provided at its lower end with an electric connector which establishes the electric connection with a bottom electric motor, is lowered by gravity into the string.
It is also known from U.S. Pat. Nos. 3,976,347 and 4,126,848 to make use of electric connectors adapted to be lowered through a string of rods at the lower end of a cable for being connected to devices located at the bottom.
Such devices cannot be used to effect operations in highly deviated wells.